The present invention is a catalyst structure and method of making, and a method of Fischer-Tropsch synthesis.
Fischer-Tropsch synthesis is carbon monoxide hydrogenation that is usually performed on a product stream from another reaction including but not limited to steam reforming (product stream H2/COxcx9c3), partial oxidation (product stream H2/COxcx9c2), autothermal reforming (product stream H2/COxcx9c2.5), CO2 reforming (H2/COxcx9c1) coal gassification (product stream H2/COxcx9c1) and combinations thereof.
Fundamentally, Fischer-Tropsch synthesis has fast surface reaction kinetics. However, the overall reaction rate is severely limited by heat and mass transfer with conventional catalysts or catalyst structures. The limited heat transfer together with the fast surface reaction kinetics may result in hot spots in a catalyst bed. Hot spots favor methanation. In commercial processes, fixed bed reactors with small internal diameters or slurry type and fluidized type reactors with small catalyst particles ( greater than 50 microns, xcexcm) are used to mitigate the heat and mass transfer limitations. In addition, one of the important reasons that Fisher-Tropsch reactors are operated at lower conversions per pass is to minimize temperature excursion in the catalyst bed. Because of the necessary operational parameters to avoid methanation, conventional reactors are not improved even with more active Fischer-Tropsch synthesis catalysts. Detailed operation is summarized in Table 1 and FIG. 1.
Literature data (Table 1 and FIG. 1) were obtained at lower H2/CO ratio (2:1) and longer contact time (3 sec or longer) in a fixed bed type reactor. Low H2/CO (especially 2-2.5), long contact time, low temperature, and higher pressure favor Fischer-Tropsch synthesis. Selectivity to CH4 is significantly increased by increasing H2/CO ratio from 2 to 3. Increasing contact time also has a dramatic favorable effect on the catalyst performance. Although reference 3 in Table 1 shows satisfactory results, the experiment was conducted under the conditions where Fischer-Tropsch synthesis is favored (at least 3 sec residence time, and H2/CO=2). In addition, the experiment of reference 3 was done using a powdered catalyst on an experimental scale that would be impractical commercially because of the pressure drop penalty imposed by powdered catalyst. Operating at higher temperature will enhance the conversion, however at the much higher expense of selectivity to CH4. It is also noteworthy that contact time in commercial Fischer-Tropsch units is at least 10 sec.
Hence, there is a need for a catalyst structure and method of Fischer-Tropsch synthesis that can achieve the same or higher conversion at shorter contact time, and/or at higher H2/CO.
The present invention includes a catalyst structure and method of making the catalyst structure for Fischer-Tropsch synthesis that have a first porous structure with a first pore surface area and a first pore size of at least about 0.1 xcexcm, preferably from about 10 xcexcm to about 300 xcexcm. A porous interfacial layer with a second pore surface area and a second pore size less than the first pore size disposed on the first pore surface area. A Fischer-Tropsch catalyst selected from the group consisting of cobalt, ruthenium, iron, nickel, rhenium, osmium and combinations thereof is placed upon the second pore surface area.
The present invention also provides a method of making a Fischer-Tropsch catalyst having the steps of: providing a catalyst structure comprising a porous support with a first pore surface area and a first pore size of at least about 0.1 xcexcm; optionally depositing a buffer layer on the porous support; depositing a porous interfacial layer with a second pore surface area and a second pore size less than said first pore size, upon the buffer layer (if present); and depositing a Fischer-Tropsch catalyst upon the second pore surface area.
The present invention further includes a method of Fischer-Tropsch synthesis having the steps of:
providing a catalyst structure having a first porous support with a first pore surface area and a first pore size of at least about 0.1 xcexcm;
a buffer layer disposed on the porous support;
a porous interfacial layer with a second pore surface area and a second pore size less than the first pore size, the porous interfacial layer disposed on the buffer layer (if present) or on the first pore surface area; and
a Fischer-Tropsch catalyst disposed on the second pore surface area; and
(b) passing a feed stream having a mixture of hydrogen gas and carbon monoxide gas through the catalyst structure and heating the catalyst structure to at least 200xc2x0 C. at an operating pressure, the feed stream having a residence time within the catalyst structure less than 5 seconds, thereby obtaining a product stream of at least 25% conversion of carbon monoxide, and at most 25% selectivity toward methane.
The present invention also includes various supported Fischer-Tropsch catalysts that are characterized by their properties. For example, a catalyst is provided that, if exposed to a feed stream consisting of a 3 to 1 ratio of hydrogen gas to carbon monoxide, at 250xc2x0 C. and a residence time of 12.5 seconds, exhibits a selectivity to methane that is greater at 24 atmospheres (contact time of 1 second) than it is at 6 atmospheres pressure (contact time of 4 seconds), even though the conversion is higher at lower pressure.
Catalytic activity is an intrinsic property of a catalyst. In the present invention, this property is defined by various testing conditions. For example, a preferred catalyst has a Fischer-Tropsch catalytic metal supported on a porous support; where the catalyst possesses catalytic activity such that, if the catalyst is placed in a tube inside an isothermal furnace and exposed to a feed stream consisting of a 3 to 1 ratio of hydrogen gas to carbon monoxide, at 250xc2x0 C., at 6 atm, at a contact time less than 5 seconds and the product stream is collected and cooled to room temperature, the selectivity to methane is less than 25%, and the carbon monoxide conversion is greater than 25%. To check whether a catalyst meets a claimed activity property requires only a test at the specified conditions.
The invention also provides a method for hydrogenating carbon monoxide, in which a feed stream containing hydrogen and carbon monoxide is passed into a reaction chamber that contains a catalyst at a temperature of at least 200xc2x0 C.; the catalyst having a supported Fischer-Tropsch catalytic metal; and collecting a product stream. In this process, heat is transferred from the reaction chamber at a sufficient rate such that, under steady-state conditions, the feed stream has: a contact time of less than about 2 seconds; a production rate of at least 1 milliliter per minute of liquid product where the liquid product is measured at 20xc2x0 C. and 1 atm or at least 1 liter per minute of gaseous hydrocarbon product of molecules having at least 2 carbon atoms; a methane selectivity of less than 25%, and a carbon monoxide conversion greater than 25%. The hydrocarbons can be saturated, unsaturated or partially oxidized; and for use as fuels are preferably saturated hydrocarbons.
The present invention further includes reactors that use any of the catalysts described herein. The invention also includes hydrocarbon fuels made by any of the methods described herein. The present invention further includes methods of hydrogenating carbon monoxide that use any of the catalysts described herein.
Advantages that may be provided by the invention include (i) at residence/contact times shorter than the prior art, higher conversions are achieved with no increase to methane selectivity; and (ii) as residence/contact times increase, conversion increases and methane selectivity decreases. Surprisingly, it has been found that carbon monoxide can be hydrogenated at short contact time to produce liquid fuels at good conversion levels, low methane selectivities and good production rates.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.